The rpm setting simply has the meter count the frequency of spark
pulses, and give a matching display based on the number of cylinders
it's told exist. So if the unit senses 2000 pulses per second, it will
display 1000 rpm for a four cylinder four cycle engine. (2000 pulses / 4
cylinders * 2 revolutions per spark).
As for why your meter is displaying correctly, it's probably already
set for 4 cylinders. On it is a switch allowing you to change from 4 to
6 and 8 cylinders. Sometimes this switch can be well hidden. There is
also a thin chance that you've got a dedicated 4 cylinder meter.
>>> "R.D. Waid" <waids@usfamily.net> 09/08/04 10:10PM >>>
Listers,
Bought an analog dwell tach on E-Bay, to check the Spit's points. I am
familiar with needing to double the 8-cylinder readings to get the
correct dwell angle. The RPM appears to be correct (matches the Spit's
mechanical tach) without doubling the value, however. The only
instructions are printed on the instrument and are not detailed. I'm
trying to understand how the thing reads rpm independently of the
number
of cylinders (if, in fact, it does?)
Bob Waid
71 Signal Red Spitfire FK4081L
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